1. Field of the Invention
This invention relates to an active type range finding device for performing a multipoint range finding operation, which is used in an automatic focus adjusting device for a cameral, etc., and particularly to an active type range finding device using a semiconductor position-sensitive detector (hereinafter referred to as "PSD").
2. Related Background Art
As one type of multipoint range finding devices has been proposed a longitudinal type multipoint range finding device as disclosed in U.S. Pat. No. 4,740,806. This type of multipoint range finding device mainly includes plural light-irradiating sources which are arranged in a direction perpendicular to a line (hereinafter referred to as "base line") connecting both centers of a light-irradiating lens and a light-receiving lens. Through the applicant's study of the above type of multipoint range finding device, the applicant has actually developed, produced and sold a camera having a built-in type of multipoint range finding device.
On the other hand, as another type of range finding device has been known a lateral type range finding device. FIG. 1 is a schematic diagram showing the operation and principle of a triangulate range finding device serving as the lateral type of range finding device. The lateral type range finding device mainly includes a light-irradiating source 4 and a PSD 20 which are disposed in a base-line direction D. Upon irradiation of a range-finding light from the light-irradiating source 4 through a light-irradiating lens 1 to a subject 3, the range-finding light is reflected from the subject 3, and irradiated through a light-receiving lens 2 onto the PSD 20. In response to the incidence of the reflected range-finding light, the PSD 20 outputs currents I.sub.1 and I.sub.2 containing information on an incident position of the light to a distance calculating circuit (not shown). The distance calculating circuit calculates a distance d to the subject 3 on the basis of the current values I.sub.1 and I.sub.2.
FIGS. 2 and 3 show examples of conventional lateral type multipoint range finding devices, respectively.
The lateral type multipoint range finding device as shown in FIG. 2 is disclosed in Japanese Laid-open Patent Application No. Sho-60-60511, and includes plural light-irradiating sources 4a to 4c for irradiating range-finding infrared-ray beams to the subject, a light-irradiating lens 1 provided on the optical axis of the light-irradiating sources 4a to 4c, a light-receiving lens 2 which is disposed away from the optical axis of the light-irradiating sources at a constant interval on the base line and has an optical axis in parallel to the optical axis of the light irradiation side, a position-sensitive detector (PSD) 20 having a photosensitive surface disposed in a direction perpendicular to the optical axis of the light-receiving lens 2, an infrared-ray light-irradiating source driving circuit 13 for driving the light-irradiating sources 4a to 4c to successively emit the infrared-ray beams therefrom, and a distance calculating circuit 14 for detecting a distance d to the subject on the basis of a distance signal corresponding to each light beam emitted from the light sources 4a to 4c.
Upon successive light-emission of the respective light-irradiating sources 4a to 4c by the light-irradiating source driving circuit 13, the light beam emitted from each of the light-irradiating sources 4a to 4c is focused on the subject 3 by the light-irradiating lens 1, and reflected from the subject 3. The light beam reflected from the subject 3 is converged by the light-receiving lens 2, and irradiated onto the PSD 20. Thereafter, an incident position of the light beam emitted from each light-irradiating source to the PSD 20 is calculated on the basis of an output current of the PSD 20 which is generated in accordance with the incidence of the light beam, and the distance d to the subject 3 is finally obtained.
FIG. 3 is a schematic diagram showing the operation principle of a lateral type multipoint range finding device having intermediate electrodes as disclosed in Japanese Laid-Open Patent Application No. Hei-1-262410. This lateral type multipoint range finding device has the similar construction as the multipoint range finding device as shown in FIG. 2, that is, includes a light-irradiating sources 4a to 4c, a light-irradiating lens 1, a light-receiving lens 2, a PSD 20, an infrared-ray light-irradiating source driving circuit (not shown) and a distance calculating circuit. The difference between FIG. 2 and FIG. 3 is that the PSD 20 of this type is newly provided with plural intermediate electrodes 7-1, 7-2 and 7-3 between a pair of electrodes at both ends of the PSD 20. The operating principle of this type of lateral multipoint range finding device is as follows. The light-irradiating sources 4a to 4c are driven by the light-irradiating source driving circuit so as to successively emit the light beams therefrom, the light beams reflected from the subject 3 are successively irradiated onto the PSD 20 to obtain an output current serving as a position-sensitive signal, and then the distance to the subject 3 is calculated on the output current value. A series of the above processes are identical to those for the lateral type multipoint range finding device as shown in FIG. 2. However, the operation of the light-receiving part is different between the devices as shown in FIGS. 2 and 3 because the construction of the PSD serving as a photosensitive portion is different therebetween. When the light beam reflected from the subject 3 is irradiated onto the PSD 20, any two electrodes through which photocurrents I.sub.1 and I.sub.2 are taken out, for example, the intermediate electrodes 7-1 and 7-2 can be freely selected in accordance with a region of the PSD to be irradiated with the light beam. In this case, the selection of the intermediate electrodes may be conducted by an electrode switching control means (not shown).
Referring to the lateral type multipoint range finding device as shown in FIG. 2 again, this device includes plural light-irradiating sources, and thus a range-finding angle is wider. In association with the widening of the range-finding angle, the distance 2L between both terminal electrodes of the photosensitive portion of the PSD is also longer. As the distance 2L between the both terminal electrodes of the PSD is longer, the range-finding precision is lowered. That is, the relationship among the distance 2L between both terminal electrodes of the PSD, a variation .DELTA.L of the light-incident position onto the PSD 20 and the output currents I.sub.1 and I.sub.2 is represented by the following equation. EQU .DELTA.(I.sub.2 -I.sub.1)/(I.sub.1 +I.sub.2)=.DELTA.L/L.
As is apparent from the above equation, as the distance 2L between both terminal electrodes of the PSD is longer, the variation .DELTA.(I.sub.2 -I.sub.1)/(I.sub.1 +I.sub.2) of a distance detection signal to the variation .DELTA.L of the light-incident position is smaller, and thus the range-finding precision is lowered. In addition, the light-incident position corresponding to the distance to the subject is varied every time the light-irradiating source is switched to another, so that a distance correction means is newly required.
On the other hand, in the lateral type multipoint range finding device as shown in FIG. 3, even if the detection range of the photosensitive portion is widened, the distance between both terminal electrodes is not lengthened because the intermediate electrodes are provided to the PSD and any two of them can be freely selected. Therefore, the variation .DELTA.(I.sub.2 -I.sub.1)/(I.sub.1 +I.sub.2) of the distance detection signal is not reduced, and thus the range-finding precision is not lowered. In addition, the intermediate electrodes may be disposed in accordance with an interval of light-incident positions so that no distance correction means is required. However, the provision of the intermediate electrodes to the PSD causes increment of a number of output terminals of the PSD, and in addition a PSD electrode switching control means and a PSD electrode selecting means are newly required. These have obstructed miniaturization of the device.
Further, in order to simplify the distance calculation for both of the PSDs used in the conventional range-finding devices as shown in FIGS. 2 and 3, an incident position of a range-finding light when the subject would be located at an infinite-point is required to be set to a just intermediate position between the two electrodes of the PSD. In this case, a half photosensitive region of the PSD (a left half region in FIGS. 1 and 2) becomes an ineffective or insensitive region where no range-finding light is detected. This is also an obstacle to the miniaturization of the device.